This invention relates generally to a method for making visible electrostatic charge patterns and more particularly to a method by which such charge patterns are developed in two colors.
In the practice of xerography, it is the general procedure to form an electrostatic latent image on a xerographic surface by first uniformly charging a photoconductive insulating surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to an original image. The selective dissipation of the charge leaves a charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern, also commonly known as an electrostatic latent image, is made visible by developing it with a toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is sometimes then fixed to the imaging surface or is transferred to a receiving sheet to which it is fixed.
This method of forming and developing charge patterns is set forth in greater detail in U.S. Pat. No. 2,297,691 to C. F. Carlson. Still other means of forming and developing electrostatic images are set forth in U.S. Pat. No. 2,647,464 to J. P. Ebert; U.S. Pat. No. 2,576,047 to R. M. Schaffert and U.S. Pat. No. 2,825,814 to L. E. Walkup.
Modern business and computer needs oftentimes make it advantageous and desirable to reproduce originals which contain two colors. It is sometimes important that the copy reproduced also contain two colors.
Accounting reports having certain information highlighted in a second color are one example of a type of document which would desirably be copied in two colors. Computer generated cathode ray tube (CRT) displays are another example in which it is somtimes desirable to reproduce an image in two colors. For instance, it is sometimes desirable that those portions of the CRT display image representing permanent forms are reproduced in a first color and those portions of the image representing variable information are reproduced in a second color.
There are known several useful methods of making copies having two colors. Some of these methods make high quality images in two colors; however, there is need for improvements in these methods. Improvements relating to the simplification of the two-color imaging process as well as to increasing image quality are especially desirable.
One method of two-color reproduction is disclosed in U.S. Pat. No. 3,013,890 to W. E. Bixby in which a charge pattern of either a positive or negative polarity is developed by a single, two-colored developer. The developer of Bixby comprises a single carrier which supports both triboelectrically relatively positive and relatively negative toner. The positive toner is a first color and the negative toner is of a second color.
The method of Bixby develops positively charged image areas with the negative toner and develops negatively charged image areas with the positive toner. A two-color image occurs only when the charge pattern includes both positive and negative polarities.
Two-color development of charge patterns created by the Tesi technique is disclosed by F. A. Schwertz in U.S. Pat. No. 3,045,644. Like Bixby, Schwertz develops charge patterns which are of both a positive and negative polarity. Schwertz's development system is a set of magnetic brushes, one of which applies relatively positive toner of a first color to the negatively charged areas of the charge pattern and the other of which applies relatively negative toner to the positively charged areas.
Methods and apparatus for making colored xerographic images using colored filters and multiple development and transfer steps are disclosed, respectively, in U.S. Pat. Nos. 3,832,170 to K. Nagamatsu et al and 3,838,919 to T. Takahashi.
U.S. Pat. No. 3,816,115 to R. W. Gundlach and L. F. Bean discloses a method for forming a charge pattern having charged areas of a higher and lower strength of the same polarity. The charge pattern is produced by repetitively charging and imagewise exposing an overcoated xerographic plate to form a composite charge pattern. Development of the charge pattern in one color is disclosed.
A method of two-color development of a charge pattern, preferably with a liquid developer, is disclosed in the commonly assigned copending application Ser. No. 587,479, filed June 16, 1975. This method requires that the charge pattern for attracting a developer of one color be above a first threshold voltage and that the charge pattern for attracting the developer of the second color be below a second threshold voltage. The second threshold voltage is below the first threshold voltage. Both the first and second charge patterns have a higher voltage than does the background.